Finding economical and efficient ways of testing the cables used in distributed networks (e.g. telephone systems) has been a problem since the first use of such networks. Historically, most prior testing methods could be represented generically by the configuration shown in FIG. 1. A test set was connected to the cable at each end. Testing was then done by exchanging signals between them. Typically, a far end set 1, i.e. telecommunications equipment located at a users location, would transmit a test signal and a near end set 2, i.e. testing equipment at the central station, would measure it. The data would then be recorded by an individual located at the near end. The test set 1, located at the far end, would be operated by an operator who had traveled to the far end, or would be controlled from the near end by an automatic device. Since typical telephone networks normally consist of many cables emanating from the near end (central telephone office) to many separate far ends (subscribers), these prior art methods normally require many separate far end test sets.
It was recognized in the prior art that traveling to the far end for testing was undesirable and, consequently, various prior art methods were employed for connecting and controlling the far end test set automatically in order to eliminate the need for a trip to the far end for testing. The principal difficulty with these prior art methods, however, was that they were both costly and complex, and they nevertheless, still had to have a test set and controller or operator waiting at each far end location.
Other systems were developed which enabled a device to be placed at the far end which is passive, but when excited from the near end returns a signal characteristic of the electrical conditions of the line. For example, Andrews et al, U.S. Pat. No. 3,526,729, discloses a system wherein a non-linear device, a diode, is placed across the line at the far end. An AC signal of constant frequency and known power is connected across the line at the near end and causes the diode to generate local harmonic signals to derive the conditions of the line at the remote location. Andrews uses a non-linear element to generate harmonics of a single signal. Andrews also discloses the use of two signals at once, wherein the harmonics of each signal is used independently.
Schimpf, U.S. Pat. No. 3,660,620 describes a similar configuration as Andrews. In addition to the first AC signal generator which generates a low frequency, high amplitude signal used to switch the alternatively into its conducting and non-conducting states, Schimpf also provides for an excitation signal generator located at the near end for generating a high frequency, low amplitude signal. These signals are amplitude modulated by the non-linear element, and the sum or difference frequencies are measured at the near end. Schimpf, uses the diode as a switch and is operated at amplitudes very close to those amplitudes used to cause ringing.
These systems have certain practical defects which render them impractical. Firstly, such systems do not isolate the far end equipment from the line while the line is being tested. This causes several possible problems with such systems.
The Schimpf design, specifically requires a signal of peak-to-peak amplitude at least equal to that of ringing voltage so that the voltage regulator (zener) diode will conduct on alternate cycles. This is undesirable, since ringing of telephones on the line would result if a signal of ringing frequency and amplitude was applied to the pair. This might also load the ringing voltage to the point that no guarantee could be given that the ring voltage would turn on the zener diode.
Also, even if a signal outside ringing frequency were used in the Schimpf invention, the electronic ringers commonly used on today's telephones would still ring. If the signal were too far outside the acceptable ringing frequency range, it might also couple with other pairs causing unacceptable interference with other calls. The Schimpf invention generates a returned frequency which is measured at the near end as a sum or difference frequency by amplitude modulation of the measurement signal frequency with the excitation signal frequency. The practical need for using a very low frequency excitation signal, since the ringing signal is confined the 16-66 Hz frequency band, places the returned signal close in frequency to the measurement signal. This causes two problems:
a) the measured signal travels the pair twice, first in the near-far direction at the measurement signal frequency, and then in the far-near direction as a returned sideband signal. Since these two signals are close in frequency, differing only by the excitation signal frequency, they experience the same attenuation in each direction. This doubles the dB loss between the measurement signal sent and the measuring signal returned, making the returned signal of very low amplitude, which is more difficult to measure with the frequency selective voltmeter, especially in a noisy environment. The measurement range of Schimpf is thus limited severely by the noise on the pair. PA1 b) Both the unattenuated measuring signal and the returned measurement signal (lower by 9 dB plus twice the line loss) are present on the test pair at the input to the frequency selective voltmeter. Since these two signals are very close in frequency, differing only by the excitation signal frequency, they require a very finely tuned filter, which adds to its complexity. PA1 c) The returned signal is directly related to the measurement signal frequency, and is thus different for every measurement frequency. This necessitates multiple filters or a tunable filter, if measurements at more than one frequency are to be made, as in slope measurement.
The present invention overcomes the disadvantages of the prior art, both as to the need for personnel and/or complex and costly equipment at the far end and employs an approach which electrically transposes the near end test set to the far end of the cable, making it appear, for all intents and purposes, to be at the far end during the test while, in reality, physically remaining at the near end. Furthermore, the present invention overcomes the practical disadvantages of the prior art systems which utilize a non-linear element at the far end. Thus, the present invention permits one test set to be used at the near end for the testing of many cables emanating from the near end, thus eliminating the need for far end test sets and the complex and costly equipment at the far end used for connection and remote control.